(i) Field of the Invention
This invention relates to a method and apparatus for the punching of holes in deformable strip and, more particularly, relates to a method and apparatus for continuously punching an array of holes in deformable strips such as lead strip for the production of lead grids for use in the manufacture of lead-acid batteries.
(ii) Description of the Related Art
Existing methods for punching material strips incur problems including low production speed, inadequate or no waste ejection, insufficient indexing precision, hole size and spacing tolerance errors, and the distortion or destruction of the final product. Previously, rotary punching of a deformable strip have employed rotary equipment having two or three shafts, each with a circumferentially distributed homogeneous male or female tool set. The rotation of adjacent shafts would be synchronized mechanically or electronically and their respective peripheral tools would interact to punch a continuous grid of holes in the strip.
Several embodiments of tooling configurations have been employed. The more traditional method of punching involved a reciprocating toolset in which male and female dies stamped one section of a material to be punched, which then had to be indexed before further punching could take place.
To punch small, closely spaced holes, attempts were made to use rotary punching technology from the metal, cardboard and plastics industry. These rotary punching methods typically create relatively large or elongated holes that are spaced quite infrequently on the material to be punched, permitting use of male/female dies that are circumferentially spaced quite far apart on the rotating shafts. U.S. Pat. No. 4,534,248 granted Aug. 13, 1985 and PCT Patent Application PTC/CA00/01288 published May 17, 2001 typify such technology. Punching produces scrap that has to be removed or ejected from the female die using some moving mechanism. These mechanically moving parts must be sufficiently robust to endure cycling, but for small, closely spaced holes there is insufficient space on a shaft for any sort of ejection mechanism, robust or otherwise. Thus, these systems have been difficult to implement and to adapt for the punching of closely spaced holes.
To counter the problems of space and moving parts, multi-stage punching with increased hole spacing was attempted. This required punching of one set of holes with one set of tooling and then indexing the punched material to another set of tooling to punch a second set of holes. Each set of tooling was on a separate shaft and, since they each had to punch fewer holes, the dies could be place farther apart, leaving sufficient room for ejecting mechanisms in the shaft. However, the problem of indexing the material to be punched from one set of tooling to the next, without compromising hole size and spacing tolerances persisted. As a result, there were frequent issues with subsequent holes not being placed the appropriate distance from the first set of holes and tolerance errors accumulated. This is confirmed in a paper entitled Rotary-Blanking published in the journal: Sheet Metal Industries, 1985, Vol. 62, Issue 2, p.134-135, in which it is acknowledged in the Conclusions that “Coordination of the rollers when several tools are mounted on the circumference is still to be solved”.
One step punching was tested in an effort to avoid the need for indexing. However, not only did the punched material not eject, but the finished product was reluctant to peel from between the male dies. Mesh had to be forcefully stripped from the male dies, which ruined the final product. Also, the entrapped waste would build up in the holes on the female shaft and cause the mechanism to seize, resulting in broken tooling. A paper entitled Rotary Blanking published Jan. 10, 1999 by Institute for Metal Forming and Casting, Technische Universitat Munchen, Germany states “As an effect of the special kinematic conditions, certain concessions concerning the quality of the sheared edges and geometrical accuracy have to be made”. This paper also states “Products with many rows of holes, in particular in combination with close hole spacings in feed direction, can only be manufactured by rotary blanking at great expense. On the one hand, large roller diameters are required to minimize the deflection of the rollers and improve the quality of the workpiece. On the other hand, the number of punches, which rises with the roller diameter and the number of rows, lets maintenance of the tools become very costly. Therefore, the most advantageous application of rotary blanking is the manufacturing of punched and pierced sheet metal products with a few number of rows and contours of a large length-to-width-ratio.”
Conventional non-rotary punching has addressed the problem of punching many closely spaced, small holes. The successful methods employ a reciprocating punch that stamps one large section of grid at a time, and then indexes the deformable strip downstream before stamping another section of the grid. This segmented approach is production-rate limiting and is relatively slow compared to rotary punching because the process is stop-and-go as opposed to continuous. These reciprocating punch presses must be robust and powerful to punch metal and the constant change in momentum due to machine oscillation creates problems with noise, precision and vibration. Indexing the strip between punches can also result in imprecision of hole placement between one set of punched holes and the next.
Indexing also has a down-stream effect on the production of mesh from lead strip because it causes a jerky motion in the movement of the lead strip. This can possibly damage the lead mesh or make it difficult to smoothly integrate the mesh into the next phase of processing.
Rotating punches that have been applied to the metal industry often rely on the shearability of a metals like steel and aluminium which do not deform plastically as much as lead and other soft materials. Even when using steel and aluminium, these rotary punches often leave burs and unclean or ragged cuts, which can result in an unacceptable accumulation of errors.
It is a principal object of the present invention therefore to provide a method and apparatus for continuous punching of deformable strip at high speed to produce a punched grid having a high tolerance, closely spaced array of holes. It is another object of the of the invention to provide two-stage rotary punch apparatus which is self-indexing for high speed production of a uniformly punched grid. Another object of the invention is the provision of a rotary punching machine which will continuously eject waste material and which readily releases the final punched product. A still further object of the invention is the provision of a rotary punching machine for punching closely spaced holes in deformable materials typified by metals and metal alloys such as lead and lead alloys, aluminum, brass, copper, steel and zinc, plastics such as Mylar™ and vinyl; cardboard and the like deformable materials.